Add buddy check system

This adds a feature to Samson that can make samson require a Buddy for certain stages. The use case here is when deploying to production stages you want to have at least 2 developers know what code is being shipped.

/cc @zendesk/samson @jwswj @pswadi-zendesk 

### TODO

- [x] fix old tests
- [ ] add new tests
- [x] add email notification for `bypass`

### RISKS
- still using the confirmation setting on stages to determine whether a stage needs buddy check.
- If this branch gets merged, the measurement of the deployment duration needs to be fixed.
I understand the sequence of events now - I saw it crop up in a commit from you - 5c87c8d - and when I couldn't locate it afterwards considered I had lost it somehow.  However, it was only an artifact present in db/schema.rb, and not in any migration file, which puzzled me last night.  OK.  I'll remove it again.